Prior surgical techniques for affixing rods to vertebrae entailed relatively long incisions to provide access to the vertebrae. Current techniques use minimally invasive surgery (MIS), in which multiple percutaneous incisions are made at chosen locations rather than a single long incision. The MIS techniques are preferable because they reduce blood loss, reduce patient morbidity and shorten recovery time and hospital stay.
Minimally invasive pedicle screw fusion involves the following basic steps. First, screws are placed percutaneously through the pedicle into the vertebral body, such as over a guide wire or K-wire, guided by imaging such as fluoroscopy. One screw may be used or two (right and left sides). Second, the screws are connected with a rod. In fusion, the rod and screws are locked together. In dynamic stabilization, the rod or rod-like device (flexible connector) is bendable, but the process of inserting the bendable rod is the same as that for fusion. For example, the rod or other flexible connector fits within the screw heads, but may also include an element (a shock absorber, a spring, etc.) that allows some motion. The variations between different minimally invasive systems mostly arise in the method of placing the rod and locking the rod with the screws through a minimal incision.
The insertion of the rod through the screw heads and locking of the rod with the screws are the steps that are currently most difficult through a minimal incision. The guidance element mentioned above, such as a guide wire or K-wire, is placed percutaneously through the pedicle. Percutaneous cannulated drills and screw taps are inserted over the guidance element/wire to prepare the tract through the pedicle and vertebral body for pedicle screw insertion. Dilating tubes and a guidance tube or a retractor system are often used to dilate and hold open the path around the guidance element through skin and muscle to reduce injury to muscle and tissue when pedicle screws and insertion tools are inserted. Pedicle screws are inserted over the guidance elements either with or without passage through a guidance tube/retractor. In order to place the rod and locking assembly into the screw heads, each screw head is associated with a tower that extends through the skin incision. The tower has to accommodate the rod and locking assemblies so it is typically larger than the maximum diameter of the screw head.
Once the towers are in place, the rod is then inserted through one of a variety of methods. In one method, two or three towers (for one or two-level fusion, respectively) are coupled together to align the towers, and the rod is swung around through a separate incision superior or inferior to the towers in a pendulum fashion. Once the rod is swung in place, locking caps are placed through the towers and tightened. In another method, the rod is inserted through one of the towers and then turned approximately 90° to capture the other screws in the other towers. Inserting the rod through the screw heads in a minimally invasive system is sometimes tedious and frustrating.
As mentioned before, a K-wire or similar guide wire or guide element (the terms being used interchangeably throughout) is used in combination with a cannulated surgical tool, such as a screwdriver, tap, bore, awl, probe, or jamshidi needle, to name some. The K-wire is inserted through the lumen (cannula) of the surgical tool and penetrates into the bone, which if not done properly can injure the patient, particularly if the K-wire encounters certain sensitive tissues. The procedures often require the use of force which can cause an otherwise properly positioned K-wire to move forward into the surgical site, which if excessive can move into contact where contact is to be avoided.
In the example of screwing a pedicle screw with a pedicle screwdriver, if the K-wire is not properly positioned or inserted, the screw can slip at the point of entry and cause the surgeon to screw the pedicle screw at the wrong orientation.